Block copolymers having the general formula --polycarbonate--G--poly ether glycol-- wherein G is a linking unit of the formula ##STR1## are known from Goldberg, U.S. Pat. No. 3,030,335. They are of substantial use in films, fibers, and as molding resins per se and in blends with other polymers. Behnke et al, U.S. Pat. No. 4,436,839 discloses their use in the production of membranes for dialysis, ultrafiltration or reverse osmosis. Unfortunately, the stability of such materials, as represented by a decrease in intrinsic viscosity after 24 hours, at 75.degree. C., is less than desirable. It is believed that this is due to the presence of aliphatic carbonate linkages. Schreckenberg et al., U.S. Pat. No. 4,217,437 discloses an approach which comprises separating the aromatic carbonate (hard) segment from the polyether (soft) segment by transesterification with carbonic acid aryl esters, and Adelmen et al., U.S. Pat. No. 4,252,922 esterifies a polyether glycol with an aliphatic diacid prior to reaction with a dihydric phenol and phosgene. Such block copolymers are disclosed to be useful in blends with other resins, affording thermoplastic molding compositions with improved resilience properties, especially at low temperatures, when aromatic polycarbonates are used as the co-blending resin. Other block copolymers blended with resins in molding compositions are known from Robeson et al., U.S. Pat. No. 4,286,075, and Fox et al., U.S. Pat. Nos. 4,510,289 and 4,511,693. In all cases, however, when polycarbonate-polyethers are used, because the criticality of avoiding any substantial content of aliphatic carbonate linkages has not been recognized, the block copolymers of the prior art are unstable, especially thermally unstable.
It has now been discovered that if block copolymers of the above general formula are provided with linking units G of the formulae ##STR2## wherein E is an unsubstituted or substituted C.sub.6 -C.sub.15 arylene or heteroarylene group, then a family of copolymers having outstanding thermal stability and ability to blend with other thermoplastics is produced. The key to such valuable materials is to avoid aliphatic carbonate linkages, and to use a family of novel terminally functional end-capped poly(alkylene glycols). The block copolymers, mixtures thereof with other polymers and the end-capped poly(alkylene glycols) are the subject matter of the present invention.